1. Field of the Invention
The present invention relates to a beam apparatus, such as a scanning electron microscope and, more particularly, to a beam apparatus capable of detecting backscattered electrons and secondary electrons separately.
2. Description of Related Art
FIG. 6 is a diagram showing an example of structure of the electron optical system of a beam apparatus, such as a scanning electron microscope. The scanning optics are omitted. The apparatus has an electron source 1 acting as a beam source. A primary electron beam 2 is emitted from the electron source 1. The beam 2 is accelerated to a desired accelerating voltage by acceleration electrodes (not shown). Then, the beam is shaped into a perfectly circular cross section by a beam-shaping aperture 7. Subsequently, the beam 2 is focused by an objective lens 4 and controlled such that the beam is brought to a focus at the surface of a sample 5 to be observed. Under this condition, the beam is scanned in two dimensions over the observed sample 5 using scanning deflectors (not shown). Produced signals, i.e., secondary electrons and backscattered electrons, are detected. A scanned image is created.
Normally, the beam-shaping aperture 7 has a function of limiting the beam current. It is possible to increase and reduce the beam current by varying the intensity of a lens (not shown) present between the aperture 7 and the electron source 1. If control is provided to increase or reduce the beam current in this way, the position of a virtual light source as viewed from the objective lens 4 varies. This makes it impossible to correctly control the angular aperture with the objective lens 4. The resolution of the scanned image depends on the diameter of the beam on the observed sample 5, but the beam diameter strongly depends on the angular aperture.
When the angular aperture has an optimum value, the smallest beam diameter is obtained, and the resolution of the scanned image is improved maximally. Accordingly, variation in the position of the virtual light source can be corrected using the condenser lens 3. As a result, the primary electron beam 2 can be brought to a focus on the observed sample 5 at the correct angular aperture.
The configuration and operation of a prior art charged-particle detector are illustrated in FIGS. 7 and 8. FIG. 7 is a diagram showing the prior art charged-particle detector and the orbit of secondary electrons. FIG. 8 is a diagram showing a prior art charged-particle detector and the orbit of backscattered electrons. In FIGS. 6, 7, and 8, identical components are indicated by identical reference numerals.
Secondary electrons 10 emitted from the observed sample 5 have various directions and various energies. The electrons travel while describing some trajectory depending on the field produced by the objective lens 4 and on the fields produced by peripheral structures. It is possible to make the secondary electrons 10 travel toward a charged-particle detector 8 within the lens by appropriately designing the objective lens 4 and peripheral structures.
In FIG. 7, the secondary electrons 10 are lower in energy than the primary electron beam 2 and, therefore, undergo a larger focusing action in the magnetic field set up by the objective lens 4. The beam 2 creates 0, 1, or more crossover points. If the orbit of the secondary electrons 10 at the location where the beam leaves the magnetic field is in a divergent direction, the beam arrives at the charged-particle detector 8, creating a scanned image signal. Usually, the position at which the secondary electron detection efficiency is optimized is determined by electron optics simulations or experiments, and the detector 8 is arranged.
In FIG. 8, backscattered electrons 9 have energies almost equal to the primary electron beam 2 and, therefore, do not undergo a large focusing action from the magnetic field produced by the objective lens 4. Accordingly, the backscattered electrons 9 arrive at the charged-particle detector 8 without creating any crossover. The detector 8 is provided with a hole 8a to permit passage of the primary electron beam 2. Where the diameter of the backscattered electrons 9 is larger than the diameter of the hole 8a, the backscattered electrons 9 are detected by the detector 8, resulting in a scanned image signal.
The angular aperture of the primary electron beam 2 is 5 to 10 mrad. It is considered that the backscattered electrons 9 have an angle of emergence that is several times larger than the angular aperture and, therefore, the scanned image signal contains a backscattered electron signal. Consequently, the output signal from the charged-particle detector 8 represents secondary electrons plus some of the backscattered electrons. The difference between secondary electrons and backscattered electrons is now described. Secondary electrons have lower energies and thus are adapted to capture an image of the surface of the observed sample. Backscattered electrons have higher energies and so penetrate into the observed sample to some extent. The backscattered electrons are adapted to grasp the composition of the sample.
A prior art apparatus of this kind has a light source-side detector and a sample-side detector on the optical axis. The detectors detect almost all secondary electrons emanating from the sample (see, for example, JP-A-2000-30654 (paragraphs 0021-0026, FIGS. 1 and 2)). Furthermore, an apparatus having first and second detectors for detecting secondary electrons or backscattered electrons produced based on the interaction between the primary electron beam and the sample is known (see, for example, JP-A-2004-221089 (paragraphs 0035-0037, FIG. 2)). In addition, an apparatus having a simple structure and capable of detecting secondary electrons and backscattered electrons from the sample separately is known (see, for example, JP-A-2000-299078 (paragraphs 0020-0033, FIGS. 1-5)). Further, an apparatus having first and second detectors and capable of detecting secondary electrons is known. The first detector detects secondary electrons emitted from the sample, while the second detector detects secondary electrons passed through a hole in the first detector. The secondary electrons are detected by combining the output signals from the two detectors (see, for example, Japanese Patent No. 3,136,353 (paragraphs 0024-0037, FIGS. 1 and 2) and JP-A-9-219170).
With the above-described prior art, it has been difficult to detect secondary electrons and backscattered electrons separately. More correctly speaking, it has been difficult to detect secondary electrons and backscattered electrons separately and simultaneously, for the following reason. The output signal from the charged-particle detector 8 is the sum of a signal indicating secondary signals and a signal indicating some of the backscattered electrons as described previously.
If it is not necessary to have simultaneity, only backscattered electrons can be detected, for example, by using a detector that is insensitive to secondary electrons (e.g., a semiconductor detector). That is, secondary electrons and backscattered electrons impinge on the semiconductor detector. Because the detector is insensitive to the secondary electrons, only the backscattered electrons can be detected as a consequence.
It is also customary to place a deflector at the position of a detector such that only secondary electrons having low energies are bent and arrive at the detector. If the deflector is operated, backscattered electrons cannot be bent because they have higher energies. Therefore, only secondary electrons can be bent and made to arrive at the detector. However, if simultaneity is obtained, the throughput will be improved easily.
The detector insensitive to secondary electrons has another disadvantage. This detector is insensitive to backscattered electrons in a case where the energies of the primary electron beam are low because, in this case, the energies of backscattered electrons are lower.